Conventionally, zinc oxide (ZnO) is transparent in a visible light region, and is a semiconductor having a relatively good property even when prepared under a low temperature. For this reason, study on ZnO has been actively carried out in recent years, so that various techniques have been reported.
For example, see Documents 1 through 3 of scientific papers. Each of the scientific papers teaches that a thin film transistor having an active layer made of ZnO operates with high performance.
(1) Document 1:    R. L. Hoffman, B. J. Norris and J. F. Wager, “ZnO-based transparent thin-film transistors” APPLIED PHYSICS LETTERS VOLUME 82, NUMBER 5, 3 FEBRUARY 2003, pp 733-735
(2) Document 2:    P. F. Carcia, R. S. McLean, M. H. Reilly and G. Nunes, Jr. “Transparent ZnO thin-film transistor fabricated by rf magnetron suputtering” APPLIED PHYSICS LETTERS VOLUME 82, NUMBER 7, 17 FEBRUARY 2003, pp 1117-1119
(3) Document 3:    Junya NISHII et al., “High Performance Thin Film Transistors with Transparent ZnO Channels” Jpn. J. Appl. Phys. Vol. 42. (2003) pp L347-L349, Part 2, No. 4A, 1 April Further, see Documents 4 through 6 of patent applications. Each of Documents 4 through 6 discloses a technique of using ZnO as a semiconductor.
(4) Document 4:    Japanese Unexamined Patent Publication Tokukai 2000-150900 (published on May 30, 2000)
(5) Document 5:    Japanese Unexamined Patent Publication Tokukai 2000-277534 (published on Oct. 6, 2000)
(6) Document 6    Japanese Unexamined Patent Publication Tokukai 2002-76356 (published on Mar. 15, 2002)
(7) Document 7    Japanese Unexamined Patent Publication Tokukaisho 63-101740 (published on May 6, 1988)
Described in Document 4 is that a transistor becomes transparent by using a transparent semiconductor such as zinc oxide for a channel layer of the transistor, and by using a transparent insulating oxide for a gate insulating layer.
Described in Document 5 is that: lattice mismatch between zinc oxide and a priming film is eliminated by selecting a material of the priming film, with the result that a semiconductor device including a thin film transistor using zinc oxide can have high performance.
Described in Document 6 is a method for doping a 3d transition metal in zinc oxide for the purpose of improving an ON/OFF ratio property and a mobility property of a transistor having a transparent channel layer made of zinc oxide or the like.
Each of the scientific papers and the documents teaches effectiveness of the transistor using zinc oxide.
However, zinc oxide is highly sensitive to an atmosphere, so that a property of the device using zinc oxide is greatly changed due to the atmosphere, as disclosed in Document 7. Therefore, a layer of zinc oxide needs to be blocked from the atmosphere by a protective layer (insulator) such that the device is put into practical use. Document 4 describes that a vertical type electric field effect transistor having the channel layer made of zinc oxide is used as a gas sensor.
No protective layer is provided in each structure of Documents 1, 2, and 3. Moreover, none of Documents 1, 2, and 3 describe an influence rendered by providing the protective layer. Meanwhile, each of Documents 4, 5, and 6 describes an example of a structure of blocking the zinc oxide layer from the atmosphere; however, none of Documents 4, 5, and 6 describe the influence rendered by providing the protective layer. Here, the aforementioned gate insulating layer corresponds to the protective layer.
For the practical use, the transistor having the active layer made of zinc oxide is required to have a stable property. An indispensable condition for attaining such a stable property is to block, from the atmosphere, the layer of zinc oxide highly sensitive to the atmosphere. For this reason, the influence rendered by providing the protective layer needs to be discussed. The following explains this.
FIG. 14(a) illustrates a transistor 50 having no protective layer. The transistor 50 has an inverse stagger structure. Specifically, a gate electrode 53 made of Ta is formed on a glass substrate 52. On the glass substrate 52 and the gate electrode 53, a gate insulating layer 54 made of Al2O3 is formed. Formed on the gate insulating layer 54 is a semiconductor layer 55 made of zinc oxide which has not been subjected to doping intentionally. Formed on the semiconductor layer 55 and the gate insulating layer 54 are a source electrode 56 and a drain electrode 57, each of which is made of Al.
FIG. 14(b) illustrates a transistor 51 provided with a protective layer. The transistor 51 has a structure similar to the transistor 50, except that a protective layer 58 made of Al2O3 is so provided as to cover a part of the semiconductor layer 55, a part of the source electrode 56, and a part of the drain electrode 57.
FIG. 15 illustrates the difference between (i) the Id-Vg property of the electric field effect transistor which has the active layer (semiconductor layer 55) made of zinc oxide and which has the protective layer, and (ii) the Id-Vg property of the electric field effect transistor which has the active layer (semiconductor layer 55) made of zinc oxide and which has no protective layer.
As shown in FIG. 15, the transistor having the protective layer has a threshold voltage greatly different from that of the transistor having no protective layer. Specifically, the threshold voltage of the transistor having the protective layer is greatly shifted to the negative side, as compared with that of the transistor having no protective layer. Such a greatly negative threshold voltage makes it impossible that the transistor is put into practical use.
The following explains why such a phenomenon occurs. That is, zinc oxide is intrinsically likely to have an oxygen hole from which free electrons are generated, so that zinc oxide is a semiconductor having the n-type conductivity. However, when the surface level of the zinc oxide layer decreases the fermi level of the surface of the zinc oxide layer, a depletion layer spreads inside the zinc oxide layer to reach the interface of the gate insulating layer serving as a channel layer, with the result that the free electrons are removed. This causes the zinc oxide layer to have a high resistance. Therefore, such a zinc oxide layer having high resistance has a small number of free electrons that are movable charges, with the result that a small gate voltage is required for removal of the free electrons. Accordingly, the absolute value of the threshold voltage becomes small. This is true when no protective layer is provided.
In the meanwhile, the surface level of the zinc oxide layer is decreased by covering the zinc oxide layer with the protective layer made of Al2O3. This can be understood based on the report of 29p-F-8 (2003 March) of Japan Society of Applied Physics 50th meeting. Specifically, it is reported that: zinc oxide and Al2O3 match well with each other, so that a defect level is small. Such reduction of the surface level causes the fermi level of the surface of the zinc oxide layer to be restored to a position determined by the density of the free electrons intrinsically included in zinc oxide. Accordingly, no depletion layer spreads inside the zinc oxide layer. With this, the zinc oxide layer is caused to have the intrinsic n-type conductivity, with the result that the zinc oxide layer has a low resistance. Therefore, a large number of free electrons exist in such a zinc oxide layer. Required for removal of such a large number of free electrons is a great negative gate voltage. Accordingly, the threshold voltage is greatly negative.
FIG. 16 illustrates the difference between (i) the resistivity of the zinc oxide layer in cases where the protective layer made of Al2O3 is provided, and (ii) the resistivity of the zinc oxide layer in cases where no protective layer is provided. As shown in FIG. 16, the resistivity of the zinc oxide layer is reduced to approximately 1/6400 by providing the protective layer. This is an evidence of the aforementioned workings.
This is the first time to find and discuss that the property of the electric field effect transistor using zinc oxide for the active layer is greatly changed by providing the protective layer as described above.
Zinc oxide is sensitive to the atmosphere, so that the protective layer is imperative for the electric field effect transistor using zinc oxide for the active layer. However, as described above, the electric field effect transistor having the protective layer has the greatly negative threshold voltage. For this reason, such an electric field effect transistor cannot be put into practical use.
The present invention is made in light of the problem, and its object is to provide (i) a semiconductor device which use zinc oxide for an active layer, and which has a protective layer for blocking the active layer from an atmosphere, and which can be put into practical use; and (ii) an electronic device including the semiconductor device.